Electrographic printer or copier devices are, for example, known from U.S. Pat. No. 6,072,977. For example, they comprise a photoconductor drum as an intermediate carrier on which is generated, by means of a laser or LED comb, a charge image of the image to be printed on a recording medium (for example a paper web). The intermediate carrier is subsequently directed to a developer station via which the charge image on the intermediate carrier is inked with toner. In a transfer printing station, the toner image is transferred from the intermediate carrier onto the recording medium and subsequently fixed in a fixing station. The printing or copying event for an image is then ended. The intermediate carrier is discharged and is then available for a new printing or copying event.
The toner image should optimally be transfer-printed from the intermediate carrier onto the recording medium without error. In the transfer printing methods used today in continuous printers in the speed range of up to approximately 1.5 m/s, print image errors primarily exist in the occurrence of voids (data loss) or brightenings in scan patterns, in blurry scan patterns and transverse stripes in scan patterns on the recording medium. In printing devices with a plurality of printing apparatuses, the 2nd printer of a twin system (or the 3rd printer of a triple system etc.) is particularly critical. This printer must print on a paper stressed (rippled, shrunk, exhibiting moisture loss) by the fixing station of the preceding printer.
Prior art in continuous printers is transfer printed with a transfer corotron; this is known, for example, from DE 197 49 386 C2. Here the recording medium is directed without additional contact pressure to the intermediate carrier in the transfer printing region and the print image is transfer-printed from the intermediate carrier onto the recording medium with the aid of the transfer corotron. The force generated on the intermediate carrier by the electrical field between transfer corotron and toner is often not sufficient in order to completely transfer the toner onto the recording medium given a rippled recording medium. Print image errors such as voids and scan blurs thereby result. Furthermore, due to the low electrostatic adhesion on the intermediate carrier, the recording medium can abruptly, uncontrollably slide through. This is shown by transverse stripes in the scanning pattern.
In order to mechanically bring the recording medium to the intermediate carrier and thereby to reduce the problems described above, additional transfer printing aids (transfer blades, pressure rollers, transfer printing jaws etc.) have been combined with a transfer corotron. However, the problems described above could not thus be completely remedied since the mechanical pressure of the recording medium on the intermediate carrier together with a transfer corotron cannot occur in the actual transfer printing region.
Furthermore, it is known (WO 02/077719 A1) to use a transfer roller as a transfer printer that presses the recording medium on the intermediate carrier in the transfer printing region. Design and function of such a transfer printing station can be learned from WO 02/077719 A1, to which reference is made and which is herewith incorporated into the disclosure. There the transfer printing station is designed such that this principle can also be used in high-speed printing or high-speed copying devices. For this, it had to be achieved that the recording medium in the transfer printing region lies securely on the intermediate carrier, so that no jumps in the relative speed between recording medium and intermediate carrier result in the printing operation and no print image errors occur on the recording medium.
Under various environmental conditions, the following disturbance variables can occur in a transfer printing station with transfer roller: partial tolerances, concentricity deviations (photoconductor, transfer roller), temperature fluctuations, contamination, wear (photoconductor, transfer roller), variable web tension in the recording medium, web tension asymmetrical with the transfer roller, varying thickness, stiffness, ripple of the recording medium. For example, it must thereby be taken into account that transfer roller and intermediate carrier must be exchanged and a perfect transfer printing must subsequently be given.